The ISR is not written in PBP. It is in assembly. The only INT_RETURN you need is at the end of the interrupt service routine - just before the ENDASM.

For 40Mhz -
Lines you need to change

DEFINE OSC 40
T0CON = %10000011 ; Turn timer on, divide by 16 prescaler

movlw 0x0B ; 100 mSec @ 40MHz
movwf TMR0H
movlw 0xDC
movwf TMR0L


If MasterClock >= 5 then ; 5 X .1 sec = .5 seconds


And no, it will not affect anything else you have running.